Membranes temperature calculations

Permeability of an FML is evaluated using the Water Vapor Transmission test.28 A sample of the membrane is placed on top of a small aluminum cup containing a small amount of water. The cup is then placed in a controlled humidity and temperature chamber. The humidity in the chamber is typically 20% relative humidity, while the humidity in the cup is 100%. Thus, a concentration gradient is set up across the membrane. Moisture diffuses through the membrane, and with time the liquid level in the cup is reduced. The rate at which moisture is moving through the membrane is measured. From that rate, the permeability of the membrane is calculated with the simple diffusion equation (Fick s first law). It is important to remember that even if a liner is installed correctly with no holes, penetrations, punctures, or defects, liquid will still diffuse through the membrane. 

Equation (2.44) indicates that for the delocalized model, the transition dipole intensity ratio is related to 0 Bm. To obtain ()Bi Bm at various temperatures, we utilize Breton s data regarding the angles between the electronic transition moments of the four BChls and the normal axis of the membrane. The calculated %BlBm are listed in Table IV. 

The oxygen activity a on the membrane surface calculated is less than Po2 measured. That means that the overall reaction is limited by the adsorption of oxygen. Figure 5 also shows that ohmic overpotantial is the dominant source of polarisation at the temperature range. The electrical power output increases with current until it reaches a point above which it decreases. In other words, there is an optimal load so that power output... 

As expected, the hydrogen flux increases with the increasing temperature as shown in Fig. 7.6 for the unsupported BCN membrane. The calculated activation energy is about 11.8 Kcal/mole. An activation energy of 12 Kcal/mole for the proton conductivity of BCN material in the presence of steam was reported in the literature [14],... 

On the other hand, the main eritieism in steam reforming membrane reactors is the necessity to impose the same operating conditions for membrane and catalyst, whereas the catalyst should be at high temperature due to reactions endothermicity while membrane temperature must not exceed a threshold for assuring its stability. Considering that for the present technology the thermal limit for Pd-based selective dense membrane has to be imposed at values lower than 800 K, the MR performance is limited as well. Calculations reported in this chapter show that, at industrial operating conditions, methane conversion in MRs is limited at 35% about. 

A porous polypropylene membrane with a water permeability coefficient of 4.2 m/s.bar is used in membrane distillation. Calculate the pure water flux for a feed temperature of 50 C and 90°C, respectively. The temperature at the permeate (distillate) side is 20°C. Neglect temperature polarization. 

Water Content and Hydration Temperature Effects. As mentioned earlier, the Dow membrane is more amorphous than Nafion 117. This allows the Dow membrane polymer matrix to adsoib more water than Nafion 117. The temperature at which the membrane is hydrated also influences the swelling of the ionomer matrix. The highest temperature used during the membrane preparation procedure controls the water content of the membrane. The water content of the membrane was calculated by dividing the weight of water absorbed by the total weight of the hydrated membrane. The water content of a membrane is primarily controlled by the inherent structure of the ionic polymer (Table I). 

Usually, PEMFC performance depends on the presence of a humid environment as well as the temperature of operation. Therefore, it is important to determine the water uptake and the stability of the prepared membranes in water at various temperatures. The temperature dependence of water uptake of the SPTES membranes was determined by the following procedure. The SPTES membranes were vacuum-dried at 100°C for 24 h, weighed and immersed in deionized water at various temperatures for 1 h. Subsequently, the wet membranes were wiped dry and quickly weighed again. The water uptake of SPTES membranes is calculated according to the method described in the Experimental section to obtain weight percent of water. The results are shown in Fig. 6.10 for comparison, the Nafion-117 membrane was also tested under the same conditions. 

Example 4.12 Calculating Crossover Losses In ref. [9], the authors noted a hydrogen crossover loss of 3.3 mA/cm for their automotive H2 PEFC applications. Calculate the mass crossover rate of hydrogen through the membrane. Also, calculate and plot the cathode activation overpotential loss at open circuit and 1 A/cm as a function of cathodic exchange current density. Assume the cathodic charge transfer coefficient at the cathode is 1.5 at a temperature of 353 K, and the fuel cell has a 50 cm geometric area. 

The methanol crossover of ZrP/Naflon 115 increased with temperature but decreased by about 50% compared with that of Naflon 115 within the entire temperature range from 30°C to 90°C. Arrhenius plots of limiting current density (/,im) of the methanol crossover as a function of temperature for Nation 115 and the composite membrane from 30°C to 90°C can be drawn. The activation energy (EJ of the methanol crossover through the membrane is calculated by fitting to the Arrhenius equation ... 

Tosti et /., 2002). Due to stability-related problems, it generally has to operate at a temperature below 500°C (Bredesen, 2008). Considering that in a catalytic reactor the temperature profiles are usually steep (De Falco et al., 2008), if a 2D model is implemented, the reactor zone temperature is calculated in every point inside the reactor, and both axial and radial profiles are available, giving the reactor designer a much more reliable assessment of the membrane temperature profile. 

Temperature profiles, methane conversion, HRF and permeated flow are shown in Fig. 14.6. Figure 14.7 shows the membrane temperature profile and the permeation driving force along the reactor. Table 14.4 shows the permeation results and the product outcome, outlining that total hydrogen permeated is lower (26%) for the counter-current configuration. The calculated HRF and methane conversion ( CH4 ) in the counter-current flow... 

In the isobutane dehydrogenation the catalytic membrane reactor allows a conversion which is twice the one observed in a conventional reactor operating under similar feed, catalyst and temperature conditions (and for which the performance corresponds to the one calculated from thermodynamics) [9]. 

Assume that 1 kmol of gas occupies 22.4 m3 at standard temperature and pressure (STP). For stage-cut fractions from 0.1 to 0.9, calculate the purity of hydrogen in the permeate, the membrane area and the fractional hydrogen recovery for a single-stage membrane. 

In practice, estimation of Laq requires information on the rate of solute removal at the membrane since aqueous resistance is calculated from experimental data defining the solute concentration profile across this barrier [7], Mean /.aq values calculated from the product of aqueous diffusivity (at body temperature) and aqueous resistance obtained from human and animal intestinal perfusion experiments in situ are in the range of 100-900 pm, compared to lumenal radii of 0.2 cm (rat) and 1 cm (human). These estimates will necessarily be a function of perfusion flow rate and choice of solute. The lower Laq estimated in vivo is rationalized by better mixing within the lumen in the vicinity of the mucosal membrane [6],... 

Calculations Membrane transport Chemical reaction networks Osmotic pressure and ion transport thermodynamics Extending the idea of chemical networks to extreme environments of temperature and pressure to discover autotrophs... 

Since active transport mechanisms require energy, the incubation temperature during the assay plays a crucial role. At 4°C, the fluidity of the cell membrane is reduced, the metabolism of the cell is downregulated, and energy-dependent transport processes are suppressed. Consequently, the amount of cell-associated target system refers mainly to the cytoadhesive fraction. In contrast, incubation at 37°C increases the fluidity of the cell membrane and the metabolic activity to an optimum, so both cytoadhesion and cytoinvasion occur at the same time. Thus, the uptake rate can be calculated from the difference in signal intensity measured upon incubation at both respective temperatures. 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

It is important to note that Vie and Kjelstrup [250] designed a method of measuring fhe fhermal conductivities of different components of a fuel cell while fhe cell was rurming (i.e., in situ tests). They added four thermocouples inside an MEA (i.e., an invasive method) one on each side of the membrane and one on each diffusion layer (on the surface facing the FF channels). The temperature values from the thermocouples near the membrane and in the DL were used to calculate the average thermal conductivity of the DL and CL using Fourier s law. Unfortunately, the thermal conductivity values presented in their work were given for both the DL and CL combined. Therefore, these values are useful for mathematical models but not to determine the exact thermal characteristics of different DLs. 

---